Mobile communication has been one of the most successful innovations in modern history. In recent years, the number of subscribers to mobile communication services has exceeded five billion and is growing fast. At the same time, new mobile communication technologies have been developed to satisfy the increasing needs and to provide more and better mobile communication applications and services. Some examples of such systems are cdma2000 1xEV-DO systems developed by 3GPP2, WCDMA, HSPA, and LTE systems developed by 3GPP, and mobile WiMAX systems developed by IEEE. As more and more people become users of mobile communication systems, and more and more services are provided over these systems, there is an increasing need of a mobile communication system with larger capacity, higher throughput, lower latency, and better reliability.
Millimeter waves often refer to radio waves with wavelength in the range of 1 mm-10 mm, which corresponds to radio frequency of 30 GHz-300 GHz. These radio waves exhibit unique propagation characteristics. For example, compared with lower frequency radio waves, millimeter waves have higher propagation loss, have less ability to penetrate objects—such as buildings, walls, foliage, and the like—and are more susceptible to atmospheric absorption, deflection, and diffraction due to particles (e.g., rain drops) in the air.
Alternatively, due to their shorter wave lengths, more antennas can be packed in a relatively small area, enabling high-gain antenna in a small form factor. In addition, these radio waves have been less utilized than the lower frequency radio waves. This also presents unique opportunities for new businesses to acquire the spectrum in this band at a lower cost. The International Telecommunications Union (ITU) defines frequencies in 3 GHz-30 GHz as SHF (Super High Frequency). Note that some higher frequencies in the SHF band also exhibit similar behavior as radio waves in the EHF band (i.e., millimeter waves), such as large propagation loss and the possibility of implementing high-gain antennas in small form factors.